In a mobile communication system, signals are transmitted from mobile stations to a base station. A digital signal is prepared for transmission by the mobile station by subjecting it to a modulation technique and using the resulting signal to modulate a carrier wave at a certain frequency. In transmission of the signal from a mobile station to the base station, it may be subject to a number of different effects, depending on the environment through which the signal passes. That environment can vary considerably, depending, amongst other things, on the distance between the mobile station and the base station, and the interference caused by buildings and other structures in the area. It is quite common for a signal received from a mobile station at the base station to comprise a number of different multi-path effects and also to be subject to noise. Processing techniques at the receiver in the base station are known to resolve the effects of the environment through which the signal passes (communication channel) and also to take into account the effects of noise. The first step at the receiver is to sample the incoming signal to take a number of digital samples from the incoming analogue signal, normally sampled at the expected bit rate of the transmitted signal. This can give rise to a DC offset component which, if not removed, could corrupt the received signal samples such that the subsequent processing would be affected. In particular, the digital signal samples may be processed by an equaliser to compensate for the effects of the channel, and the known equalisers do not assume that a DC offset will be present. There are other sources that may introduce a DC offset and the magnitude of the DC offset may vary. It is an aim of the present invention to obtain a reliable estimate of the offset magnitude that needs to be removed from the signal, without degrading the performance too much in the case that in fact no DC offset has been introduced.
In current base stations, a possible DC component is removed from the received signal by digital signal processing means. This is performed by removing the mean signal level from the real and imaginary component separately. Thus, the received signal y is considered to comprise both a real component and an imaginary component which are handled separately. A mean value (Ey) can be calculated over a whole burst to improve reliability. However, the transmitted data itself can cause the average value to have a false mean value, even in the case where there is no actual DC offset. This clearly degrades the performance of the subsequent digital signal processing.